lib: add IPv4 single hop BFD (RFC5881) support.
lib: add IPv4 single hop BFD (RFC5881) support.

Signed-off-by: Wei-Li Tang <5d755dd15206b0463ba6e773187c465627300b40@xinguard.com>
Signed-off-by: FUJITA Tomonori <93dac1fe9c4b2a3957982200319981492ad4976e@lab.ntt.co.jp>

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